Digital printing apparatus, such as electrophotographic "laser" printers and ink-jet printers, are well known. In color digital printing apparatus, a small set of colorants, typically black, yellow, magenta, and cyan, are selectably placed in different areas on a print sheet, and small areas of each primary colorant are allowed to blend together optically, to create a large range, or gamut, of colors which would be apparent to an observer.
To obtain a large gamut of colors from a small number of colorants, a variety of halftoning techniques can be applied. One technique, known as "error diffusion," is well known. In error diffusion, in order to print a small area of a desired "source color," the source color is located in color space relative to the locations of the primary colorants, such as yellow, magenta, and cyan; the colorant which is closest to the target color in color space is then selected for one pixel area. However, the error, essentially meaning the Euclidean distance in color space between the source color and the selected colorant, is recorded, and is in effect distributed or diffused to the image data of neighboring pixel areas. In brief, this diffusion of each error to neighboring pixel areas will influence the decision of which primary colorant to use in those neighboring pixel areas. The overall effect is, over a reasonably large number of pixels, an optical blend resulting in the desired source color.
Recently, particularly in the technology of ink-jet printing, there has been developed a hardware option in which selectably available colorants are provided beyond the usual pure primary colorants, such as cyan, magenta, and yellow. In some designs, there may be available colorants of additive colors, such as red, blue, and green, in addition to the subtractive colors; in other designs, there may be available for selection a colorant which represents a lighter or "diluted" version of another primary color, such as a light cyan, which is 50% lighter than regular cyan. The use of such additional colorants can enhance and/or enlarge the available gamut associated with a particular apparatus. One particular additional colorant, which will be the subject of the embodiment of the present invention described below, is, in addition to a pure black K colorant, a mid-gray (MG) colorant, which in effect is a 50% dilution of black ink. Selectable use of the mid-gray ink will, of course, be helpful in the creation of monochrome halftones (such as black-and-white photographs) and also for the creation of non-saturated colors.
These additional colorants, such as light cyan or mid-gray, can be considered "intra-gamut" colorants; which is to say, whereas pure colorants such as cyan or magenta define a gamut and are therefore disposed in color space at the edge or surface of a gamut, these intra-gamut colorants are disposed in color space inside the gamut defined by other colorants. While this is useful for obtaining accurate representations of colors, such as "pastels," which are near the white or gray areas of a gamut, use of such intra-gamut colorants can interfere with the error-diffusion colorant selection process. If an error-diffusion selection process occasionally "decides" that an intra-gamut colorant, such as mid-gray, is desirable for a particular pixel in an image, the resulting error from selection of this intra-gamut color, when diffused to influence the selection of colorants for neighboring pixels, may require the selection of colors which are out of the gamut and which would be physically impossible to obtain with the available colorants. In many situations, selection of a colorant from inside the gamut can lead to errors which require selection of colorants outside the gamut, which are not available.
The present invention is directed to an error-diffusion system, specifically for use in apparatus wherein intra-gamut colorants, such as mid-gray, or lighter or diluted versions of primary color colorants, are available.